Since Andre Geim and KonstaninNovoselof from University of Manchester in UK successfully stripped pyrolytic graphite out and observed graphene in 2004 (Novoselov K. S.; Geim, A. K.; Morozov, S. V; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666-9), the investigation of new carbon materials has been remaining a hot topic in relevant areas. The success of stripping graphene out breaks the prediction about thermal instability of two-dimensional crystal theoretically, and brings about possibilities for more investigations and explorations in new fields.
Perfect graphene is supposed to own ideal two-dimensional structure, which consists of hexagonal lattice. Every single carbon atom is combined with other three carbon atoms by σ-bond in the direction of lattice plane, and non-bonding electrons serves as π-electrons, forming π-orbit system vertical to the lattice plane. The π-electron could move randomly in the plane, which enables graphene to own fine electrical conductivity and sustain electric current whose density is six orders of magnitude more than copper. Graphene also owns record-breaking thermal conductivity. Thermal conductivity of pure graphene could reach 2000-4000 W/(m·K), and it also has excellent strength and large surface area. Besides, the special structure of graphene provides unique energy band structure and enables it with half integer quantum hall effect and perfect tunneling effect, as well as electrical conductivity that would never fade away. The special characteristics mentioned above guarantee graphene a promising prospect of application in fields of materials and electronic circuits. Therefore, is of great demand for the synthesis of graphene.
There're two traditional ways to synthesis graphene, which are physical method and chemical method respectively. Properties of graphene obtained through the two methods are different from each other. Physical methods include mechanical stripping, electric arc discharge, ultrasonic dispersion etc. Graphene layers obtained through physical methods are comparatively intact, but there're problems like low productivity, uncertainty of quality, command for special equipment and high cost. While chemical methods include bottom up organic synthesis, oxidation-reduction process, solvothermal synthesis and chemical vapor deposition. Equipment and raw materials are strictly required for organic synthesis method, so it's difficult to put into mass production in this way. Production quality isn't stable for solvothermal method, thus the average quality is poor. Chemical vapor deposition method costs too high and cannot achieve scale production. Among all those methods, only oxidation-reduction process can work without special equipment, and quality of graphene obtained through this method is stable. Thus it's the most suitable way for industrialized production. But there are two main questions about the graphene which run through oxidation-reduction process. Firstly, the intense oxidation-reduction reaction breaks the 6-membered carbocyclic rings in the graphene sheets, which end up with so called defects, and leads to the affection and interference of graphene material's property. Secondly, the graphene material obtained can't be reduced completely, which leads to the oxygen residual on the graphene sheets, which can also affect the property of graphene material. Therefore the graphene obtained is so called “reduction-oxidation graphene”, By comparison the graphene material obtained by chemical vapor deposition is of less defects, and have no oxygen residual. Therefore in certain application range, this kind of graphene serve our needs better, which can be called “graphene” better.
As to reduced graphene oxide, there are couple of surface defects: 1. Stone-Wales(SW) topological defect, which is, the carbon atoms rearrangement happened in the two hexagonal rings, leads to the break of this structure then formed as a penta cyclic and a seven-membered-cycle. 2. Vacancy defect, which means one or more carbon atoms missed in the honeycomb pattern of graphene sheets, make the vacancy happened in the graphene. The vacancy can be classified as single vacancy defects and double vacancy defects. 3. Doping heteroatoms, which are, the carbon atoms of graphene adsorb the heteroatoms or be substituted by heteroatoms. As such of these vacancies, it makes the difference between chemical preparation and physical preparation, included the size of surface area, the scale of carbon and oxygen and energy gaps in the energy band structure exist or not and so on.
The properties of reduced graphene oxide are quite different from the graphene obtained by chemical vapor deposition, which is caused by the vacancies, therefore, we can design some catalytic steps for surface reconstruction, to make vacancies of graphene eliminated, meanwhile the residual oxygen can be removed, and the reduced graphene oxide obtained by chemical method turn out into high quality graphene, to reach the goal of preparing graphene of high quality at low cost, which is the research that is imperative.
However, the researches on repairing and reconstruction of graphene deficiencies are less reported, there is a large blank in this research; on the other hand, the reports focus on repairing carbon atoms of graphene by chemical vapor deposition method so far internationally, but the traditional organic synthesis method to repair graphene is less reported.